A constitution has been known in the art that employs a viscous coupling as a roll damper (See Patent Document 1, for instance). The roll damper is such that a plurality of doughnut-shaped outer plates fixed to the inner peripheral surface of a hollow casing are alternately overlapped with a plurality of doughnut-shaped inner plates fixed to the outer peripheral surface of a rod (shaft) in a hollow casing, which is rotatably installed in relation to the rod, and also a silicone oil is encapsulated in the casing. And a damping force is created by the viscosity resistance of the encapsulated oil.    [Patent Document 1] Japanese Patent No. 2803870.    [Patent Document 2] Japanese Patent Application Publication No. Hei10-109529.    [Patent Document 3] Japanese Patent Application Publication No. Hei10-109528.    [Patent Document 4] Japanese Patent Application Publication No. Hei02-62431.